Desorption-diffusion model and lost gas quantity estimation of coalbed methane from coal core under drilling fluid medium

Abstract

The differences of coalbed methane (CBM) desorption-diffusion from coal drilling-core under various drilling fluid medium are not considered in the present calculating methods of lost CBM quantity, which leads possibly to the inaccuracy of CBM quantity in coal seam. Here we took the desorption of CBM from coal core under drilling fluid medium as a pressure-swing process, and based on the Langmuir equation and Fick-first law, established the desorption-diffusion model and numerical modeling method of lost gas (including free CBM) calculation in coal core under various drilling fluid mediums through physical simulation test and by considering comprehensively primary factors. The results showed that the physical simulated t-Qt curves can be rightly fitted by the numerical modeling data, which indicated the ultimate desorption quantity from the numerical modeling was adjacent to that from the physical simulation as a whole. It was found that the lost CBM quantity from the modeling method was generally higher than that from the direct method when lost time was relatively long. Thus, we suggest that it is necessary to emend the active China national standard through further investigation, since the lost CBM quantity from coal drilling-core was generally underestimated using the method in the current standard. © 2010 Science China Press and Springer-Verlag Berlin Heidelberg